/Engine/Source/Runtime/SlateCore/Public/Application/SlateApplicationBase.h
Gets whether or not a particular widget has the specified users focus, and if so the type of focus.
The optional will be set with the focus cause, if unset this widget doesn't have focus.
[TOptional](API\Runtime\Core\IO\TOptional)< EFocusCause > HasUserFocus ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< const [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > Widget, int32 UserIndex ) const